# All you need to know about Carpenter's Square - 21

Diagram 21.1: Black to play

Let us discuss this corner shape seemingly weakened by . Suppose white just plays , how can black respond?

Diagram 21.2: White fails

is incorrect, but is wrong too. is the key point and black is alive.

Diagram 21.3: Black fails

occupies the key point instead. is the most tricky answer (clearly or not working). Although is a good move (a common tesuji to avoid those big-eye killing shapes), it doesn't work here......

Diagram 21.3: Black fails (continued)

is a calm move. Black is killed.

Diagram 21.4: Solution

It becomes clear now that is the key point. If white plays , black has a number of forcing moves, followed by .

Diagram 21.5: Solution (continued)

can be a tesuji sometimes. But because of the external liberty at 'a', black can use Oshi-tsubushi tesuji to kill the white stones.

Diagram 21.6: Black fails

also looks like a key point, but so is . If black chooses , white can form almost a picnic ko. Black clearly fails.

Diagram 21.7: Black fails too

here is better. After there is just enough room for black to make two eyes. Black is successful in a pure life & death sense. But comparing to Diagram 21.4, black is almost 10 points worse off so I cannot accept this as an alternative solution.

Diagram 21.8: White fails

is slightly more challenging. is incorrect. Black can play and the corner is completely alive.

Diagram 21.9: Same as Diagram 21.6

Apparently is the key point. This results the same as Diagram 21.6.

Diagram 21.10: Black fails

doesn't work and the corner is killed by .

Conclusion: Diagram 21.4 is the only right solution. Luckily it is not hard to memorise that: is almost always the key point for Carpenter's Square shapes.